As noted hereinabove, the present invention is particularly concerned with solution well mining, such as is employed in mining uranium, wherein a leaching solution is utilized. In general, such an operation employs a plurality of wells including an injection well into which the leachant is pumped and at least one production or recovery well which is located some distance from the injection well. The wells extend into a mineralized zone, i.e., a substratum containing the mineral sought to be recovered, and the leaching solution passes through the zone from the injection well to the recovery well and carries with it the mineral to be recovered, viz., uranium.
Such uranium leaching wells are conventionally cased with polyvinyl chloride and cemented to depth beyond the mineralized zone. The grouted casing in the mineralized zone is then removed with a mechanical reaming device and a so-called well screen is placed in the mineralized zone of the well. This well screen is designed to permit only essentially sand free liquid to flow into the well bore. It will be appreciated that sand must not be permitted to enter the well bore because such sand will drastically accelerate the wear of the downhole submersible centrifugal pumps used to lift the leachant to the surface. It will be understood that the process described above, wherein the well is underreamed and a well screen used to prevent the ingress of sand, is relatively expensive and time consuming.
A further, more general problem associated with uranium leaching wells is that many wells exhibit below standard injectivity so that little or no mineral recovery is achieved. Although well stimulation methods such as "acidizing" are available, these simply are not effective in many instances.
As will be discussed hereinbelow, the present invention concerns the use of a water jet perforator in uranium solution mining and like applications. It is noted that casing perforators using fluid jets have been used for cutting steel casings in oil wells. Although this art is not thought to be relevant, reference is made to U.S. Pat. Nos. 2,638,801 (Klassen et al.); 2,302,567 (O'Neill); 2,315,496 (Boynton); 3,066,735 (Zingg); and 3,130,786 (Brown et al.) which disclose various forms of perforating apparatus for this purpose. Characteristically, these apparatus employ a drill fluid which contains an abrasive such as sand or grit and/or a chemical used in enchancing perforation. The use of an abrasive jet presents operational problems particularly with regard to wear. Moreover, the pressures used are generally substantially lower than employed in accordance with the present invention.